US10724114B2 - High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet - Google Patents

High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet Download PDF

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US10724114B2
US10724114B2 US15/740,055 US201615740055A US10724114B2 US 10724114 B2 US10724114 B2 US 10724114B2 US 201615740055 A US201615740055 A US 201615740055A US 10724114 B2 US10724114 B2 US 10724114B2
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steel sheet
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US20180202016A1 (en
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Takafumi Yokoyama
Hiroyuki Kawata
Riki Okamoto
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Nippon Steel Corp
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/005—Ferrite
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet.
  • One of methods for forming vehicles or members of automobiles using such high-strength steel sheets is a bending method, such as press forming.
  • the bendability tends to deteriorate as the strength of a steel sheet is increased. Therefore, there has been a problem in that when a high-strength steel sheet is subjected to bending, fissures (cracks) occur within the steel sheet at a deformed part.
  • Patent Document 1 discloses technology that relates to a hot-dip galvanized steel sheet or a galvannealed steel sheet having a tensile strength of 1180 MPa or more in which the bendability is improved by dissolving Zn in a surface layer portion of the steel sheet and softening the surface layer portion of the steel sheet, and furthermore, making the metal micro-structure constituting the steel sheet a micro-structure that mainly consists of martensite and bainite.
  • Patent Documents 2 and 3 disclose technology relating to an ultra-high strength cold-rolled steel sheet that, by controlling the atmosphere during continuous annealing to an oxidizing atmosphere to cause a decarburized layer to form on an outer layer of a steel sheet, improves the bendability by separately forming a soft layer that mainly consists of ferrite as the outer layer and a hard layer that mainly consists of martensite and bainite as an inner layer.
  • Patent Document 4 discloses technology relating to a high-strength cold-rolled steel sheet that, after heating a steel sheet, sprays water onto the surface to cool a surface layer portion and thereafter uniformly cools from the outer layer of the steel sheet to the interior to thereby vary the cooling patterns for the surface layer portion and the interior of the steel sheet and separately form a soft layer that mainly consists of ferrite in the outer layer and a hard layer that mainly consists of a low-temperature transformation phase in an inner layer to thus improve the bendability.
  • Patent Document 1 in order to cause Zn to dissolve in a surface layer portion of a steel sheet, it is necessary to make the heating temperature when annealing a high temperature of the Ac 3 point+50° C. or more. This is not preferable since it hastens the occurrence of damage to the furnace body of the continuous annealing furnace.
  • the atmosphere during annealing is made an oxidizing atmosphere for the purpose of decarburization, and an internal oxidized layer of an alloying element such as Mn and Si is formed in the outer layer of the steel sheet. Because of the existence of the internal oxidized layer, in some cases the fatigue strength decreases significantly, and consequently room for improvement still remains.
  • Patent Document 4 since it is necessary to perform water spraying onto the surface of a cast piece, there is a problem in that it is difficult to apply the technology to heat treatment on a hot-dip galvanization line.
  • An objective of the present invention is to solve the aforementioned problems and provide a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet that are excellent in bendability.
  • the present invention was conceived to solve the issues described above, and the gist of the present invention is a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet which are described hereunder.
  • a high-strength cold-rolled steel sheet having a chemical composition consisting of, by mass %,
  • V 0 to 0.50%
  • B B content (mass %) contained in steel sheet
  • Bs B content (mass %) present as a solid solution from a surface down to a depth of 30 rpm of the steel sheet;
  • V 0.001 to 0.50%
  • a steel micro-structure at a position from a surface down to a depth of 30 ⁇ m of the steel sheet is, in area %:
  • polygonal ferrite 10 to 95%
  • the balance one or more kinds selected from martensite, bainite and retained austenite,
  • a steel micro-structure at a position of 1 ⁇ 4 thickness of the steel sheet is, in area %:
  • the balance one or more kinds selected from martensite, bainite and retained austenite,
  • polygonal ferrite 30 to 95%
  • a steel micro-structure at a position from the surface down to the depth of 30 ⁇ m of the steel sheet is, in area %:
  • polygonal ferrite 10 to 80%
  • a steel micro-structure at the position of 1 ⁇ 4 thickness of the steel sheet is, in area %:
  • polygonal ferrite 20% or less
  • a tensile strength is 980 MPa or more, and a ratio R/t between a sheet thickness t and a minimum bending radius R is 2.5 or less.
  • a tensile strength is 1180 MPa or more, and a ratio R/t between a sheet thickness t and a minimum bending radius R is 3.5 or less.
  • a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet that are excellent in bendability can be obtained.
  • FIG. 1 is a view for describing the positional relationship between a nozzle and a steel sheet during a descaling process.
  • the present inventors conducted intensive studies in order to obtain a high-strength cold-rolled steel sheet that is excellent in bendability. As a result, the present inventors discovered that the bendability of a steel sheet can be improved without lowering the strength of the steel sheet by controlling how B, which is a hardenability element, is present to mainly a precipitation state in a surface layer portion of the steel sheet and to mainly a solid solution in the interior of the steel sheet.
  • the present inventors discovered that it is possible to suppress the occurrence of a deterioration in the strength and to improve the bendability by making a ratio between the B amount that is present as a solid solution and the total B amount contained in the steel 0.50 or less from the steel sheet surface down to a depth of 30 ⁇ m (in the following description, this area is also referred to as a “surface layer portion”), and making the aforementioned ratio more than 0.50 at a position of 1 ⁇ 4 thickness of the steel sheet (in the following description, this position is also referred to “interior”).
  • the present inventors discovered that in order to control the presence state of B so as to satisfy the aforementioned conditions, it is effective to make the scale thickness immediately after coiling of a hot-rolled coil a thickness that is within a predetermined range, and to adjust the cooling conditions after coiling the hot-rolled coil.
  • the C content is made 0.050 to 0.40%. From the viewpoint of increasing the strength, preferably the C content is made 0.080% or more. Further, from the viewpoint of suppressing a deterioration in the press-formability and weldability, preferably the C content is made 0.25% or less.
  • Si silicon
  • the Si content is made 0.01% to 3.0%.
  • the Si content is made 0.10% or more, and more preferably is made 0.20% or more.
  • the Si content is made 2.0% or less, and more preferably is made 1.50% or less.
  • Mn manganese
  • the Mn content is made 1.0 to 5.0%.
  • the Mn content is made 1.5% or more.
  • the Mn content is made 3.0% or less.
  • Al (aluminum) is contained in an amount of at least 0.001% for deoxidation of the steel. However, if Al is contained in an excessive amount, the effect is saturated, and the excessive amount not only leads to an increase in the cost but also raises the transformation temperature of the steel and increases the load during hot rolling. Therefore, the sol. Al content is made 1.0% or less. Preferably the sol. Al content is made 0.005% or more, and is preferably made 0.5% or less.
  • Ti immobilizes N as TiN in the steel to thereby suppress formation of BN that becomes a factor that reduces hardenability. Further, Ti refines the austenite grain size during heating to enhance toughness. On the other hand, if Ti is contained in an excessive amount, the ductility of the steel sheet decreases. Therefore, the Ti content is made 0.005 to 0.20%. The Ti content is preferably made 0.010% or more, and is preferably made 0.050% or less.
  • B (boron) segregates at austenite grain boundaries or ferrite/austenite grain boundaries when heating the steel sheet and stabilizes the grain boundaries to thereby enhance the hardenability of the steel, and therefore B is an essential element in the present invention.
  • the B content is made 0.0005 to 0.010%.
  • the B content is preferably made 0.0010% or more, and is preferably made 0.0050% or less.
  • P phosphorus
  • the P content is made 0.1% or less. More preferably, the P content is made 0.05% or less. However, if the P content is reduced extremely, the dephosphorization cost will be high, and therefore from the viewpoint of economic efficiency it is preferable to make the lower limit 0.001%.
  • S sulfur
  • the S content is made 0.01% or less.
  • the S content is preferably made 0.005% or less, and more preferably 0.002% or less.
  • the desulfurization cost will be high if the S content is reduced extremely, from the viewpoint of economic efficiency it is preferable to make the lower limit 0.0005%.
  • O oxygen
  • the O content is preferably made 0.01% or less, and more preferably 0.005% or less.
  • the lower limit of the O content is preferably 0.0001%.
  • N nitrogen
  • nitrogen is an element contained as an impurity. If the content of N is more than 0.01%, the N forms coarse nitrides in the steel and deteriorates bendability and hole expandability. Therefore, the N content is made 0.01% or less. The N content is preferably made 0.005% or less. However, if the N content is reduced extremely, the denitrification cost will be high, and therefore from the viewpoint of economic efficiency the lower limit is preferably made 0.0005%.
  • one or more elements selected from Cr, Mo, Ni, Cu and Sn is contained in an amount of 0.001% or more, and more preferably in an amount of 0.05% or more.
  • Nb (niobium), V (vanadium) and W (tungsten) are carbide-forming elements and are effective elements for enhancing the strength of the steel sheet, and may therefore be contained according to need. However, if any of these elements is contained in an excessive amount, the effect is saturated and it results in an increase in cost. Therefore, the Nb content is made 0.20% or less, and the content of V and the content of W are each made 0.50% or less. The Nb content is preferably made 0.10% or less, and the content of V and the content of W are each preferably made 0.30% or less.
  • one or more elements selected from Nb, V and W is contained in an amount of 0.001% or more, and more preferably in an amount of 0.005% or more.
  • Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium) and REM (rare earth metal) are elements that contribute to finely dispersing inclusions in the steel
  • Bi (bismuth) is an element that reduces micro-segregation of substitutional alloying elements such as Mn and Si in the steel. Because these elements each contribute to improving the bendability of the steel sheet, the respective elements may be contained according to need. However, if an excessive amount of these elements is contained, the elements will cause the ductility to deteriorate. Therefore, the content of each of Ca, Mg, Bi, Zr and REM is made 0.01% or less, and the Sb content is made 0.10% or less. The content of each of Ca, Mg, Bi, Zr and REM is preferably made 0.006% or less, and the Sb content is preferably made 0.080% or less.
  • one or more elements selected from Ca, Mg, Bi, Sb, Zr and REM is contained in an amount of 0.0001% or more, and more preferably in an amount of 0.0010% or more.
  • the term “REM” refers to a total of 17 elements that are Sc, Y and the lanthanoids, and the aforementioned content of REM means the total content of these elements. Note that, in industrial use the lanthanoids are added in the form of misch metal.
  • the balance is Fe and impurities.
  • impurities refers to components which, during industrial production of the steel sheet, are mixed in from raw material such as ore or scrap or due to various factors in the production process, and which are allowed within a range that does not adversely affect the present invention.
  • Bs B content (mass %) present as a solid solution from the surface down to a depth of 30 ⁇ m of the steel sheet
  • sol. Bs/B is more than 0.50, the hardenability of the surface layer portion will increase excessively and it will therefore not be possible to ensure bendability.
  • the value of sol. Bs/B is preferably made 0.30 or less, and more preferably is made 0.20 or less.
  • the steel sheet interior it is important that B is caused to be present as mainly a solid solution. If the value of sol. Bq/B is 0.50 or less, the hardenability of the steel sheet interior will decrease and it will therefore not be possible to secure the required strength.
  • the value of sol. Bq/B is preferably made 0.65 or more, and is more preferably made 0.80 or more.
  • the values of sol. Bs and sol. Bq are determined by calculating, at the respective predetermined positions thereof, the B amount consumed as a precipitate by measuring the mass of boride in the steel by an electrolytic extraction residue method, and thereafter deducting the calculated B amount from the B content contained in the steel.
  • Non-Patent Document 1 a technique disclosed in Non-Patent Document 1 is used as a method for determining the precipitated B amount by the extraction residue method.
  • the steel micro-structure of the steel sheet of the present invention in order to compatibly achieve both strength and bendability, it is preferable to adjust the respective steel micro-structures of the surface layer portion and the interior of the steel sheet. Specifically, from the surface down to a depth of 30 ⁇ m of the steel sheet, it is preferable to make the area fraction of polygonal ferrite 10 to 95% and to make the balance one or more kinds selected from martensite, bainite and retained austenite, and at a position of 1 ⁇ 4 thickness of the steel sheet, it is preferable to make the area fraction of polygonal ferrite 60% or less and to make the balance one or more kinds selected from martensite, bainite and retained austenite.
  • the aforementioned martensite includes as-quenched martensite and tempered martensite subjected to tempering in addition to quenching. Because as-quenched martensite is brittle in comparison to tempered martensite, it is liable to become the origin of fractures when subjecting the steel sheet to plastic deformation such as bending. Therefore, in order to secure the desired bendability, in each of the surface layer portion and the interior of the steel sheet, it is preferable to make the proportion of tempered martensite to the entire martensite 50% or more, and more preferably 70% or more.
  • the area fraction of polygonal ferrite in the surface layer portion of the steel sheet 30 to 95% and to make the area fraction of polygonal ferrite in the interior of the steel sheet 10 to 60%.
  • the area fraction of polygonal ferrite in the surface layer portion of the steel sheet is more preferably 50 to 90%, and an area fraction of 70 to 90% is further preferable. Further, the area fraction of polygonal ferrite in the steel sheet interior is more preferably 20 to 40%.
  • the area fraction of polygonal ferrite in the surface layer portion of the steel sheet 10 it is preferable to make the area fraction of polygonal ferrite in the surface layer portion of the steel sheet 10 to 80% and, in the interior of the steel sheet, to make the area fraction of polygonal ferrite 20% or less, to make the area fraction of martensite 50% or more, to make the area fraction of bainite 40% or less, and to make the area fraction of retained austenite 20% or less.
  • the area fraction of polygonal ferrite in the surface layer portion of the steel sheet is more preferably 30% or more, and further preferably is 50% or more.
  • the area fraction of polygonal ferrite is more preferably less than 10%, and further preferably is less than 5%, and the area fraction of martensite is more preferably 70% or more.
  • the steel micro-structure in the present invention is measured by the method described hereafter.
  • a section in the rolling direction of the steel sheet is cut out, and the steel micro-structure is revealed by using a nital solution.
  • a position from the surface to a depth of 30 ⁇ m and a position of 1 ⁇ 4 thickness of the steel sheet are photographed using a scanning electron microscope (magnification: ⁇ 5000, 5 visual fields).
  • the area fractions of polygonal ferrite, bainite, martensite and tempered martensite are calculated by the point counting method based on the obtained micro-structure photographs.
  • the area fraction of retained austenite is determined by calculating the area of a region having an FCC structure by the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method.
  • the steel sheet according to the present invention has both high strength and excellent bendability.
  • the tensile strength is 980 MPa or more and the ratio R/t between the sheet thickness t and the minimum bending radius R is 2.5 or less.
  • the tensile strength is 1180 MPa or more and the ratio R/t between the sheet thickness t and the minimum bending radius R is 3.5 or less.
  • the tensile strength is more preferably 1470 MPa or more.
  • the minimum bending radius R is evaluated by a V block method in accordance with a bending test specified in JIS Z 2248. Specifically, a strip specimen in a direction (width direction) orthogonal to the thickness direction and the rolling direction is cut out, the bending radius is varied to perform 90-degree V bending, and the smallest bending radius at which cracking does not occur is taken as the minimum bending radius.
  • the high-strength cold-rolled steel sheet according to the present invention described above may have a hot-dip galvanized layer on the steel sheet surface.
  • the corrosion resistance is improved by providing a hot-dip galvanized layer on the steel sheet surface.
  • the hot-dip galvanized layer may be subjected to alloying. Because Fe is incorporated into the hot-dip galvanized layer by the alloying treatment, the alloyed hot-dip galvanized layer is excellent in weldability and coating properties.
  • performing plating of an upper layer on the hot-dip galvanized layer may be performed for the purpose of improving coating properties and weldability.
  • various kinds of treatment such as a chromate treatment, a phosphate treatment, a lubricity enhancing treatment, or a weldability enhancing treatment may be performed on the hot-dip galvanized layer.
  • the high-strength cold-rolled steel sheet can be produced using a method that includes the processes described hereunder.
  • a slab is heated to a temperature of 1150° C. or more.
  • the slab heating temperature 1150° C. or more it is preferable to make the slab heating temperature 1150° C. or more to promote melting of borides.
  • a steel slab used is preferably cast by a continuous casting process from the viewpoint of producibility, the steel slab may also be cast by an ingot-making process or a thin slab casting process. Further, the cast slab may be cooled temporarily to room temperature or may be sent directly to a heating furnace without being cooled to room temperature.
  • the heated slab is rolled so that the total rolling reduction in a temperature range from 1050 to 1150° C. is 50% or more. If the total rolling reduction in the aforementioned temperature range is less than 50%, there is a risk that recrystallization during hot rolling will be insufficient and this will lead to heterogenization of the micro-structure of the hot-rolled sheet.
  • the total rolling reduction from a temperature of 1050° C. or less to before the final pass of the finish rolling process (final finishing pass) is made 60 to 95%, and the rolling reduction in the final finishing pass is made 10 to 30% and the temperature for the final finishing pass is made 850 to 1000° C.
  • the steel sheet surface is subjected to descaling one or more times during the process from rough rolling to finish rolling.
  • the final descaling temperature is made 950 to 1100° C. If the final descaling temperature is less than 950° C., because the growth of scale after descaling is suppressed, it will be difficult to control an average thickness tsc of the scale of the steel sheet immediately after coiling of a hot-rolled coil, described later, to be within a desired range. On the other hand, if the final descaling temperature is more than 1100° C., because scale will grow excessively after the final descaling, there is a risk that scale will peel off during rolling and that defects in the appearance of the steel sheet will arise due to scale biting. Although the total number of times to perform descaling is not particularly defined, it is preferable to perform descaling two or more times to suppress the occurrence of defects in the appearance of the steel sheet caused by biting of scale that peeled off during rolling.
  • a distance (D) from the nozzle to the steel sheet, and an angle ( ⁇ ) formed between the nozzle and the sheet thickness direction of the steel sheet can also be important factors.
  • the desired tsc can be obtained by making the descaling water pressure 10 to 20 MPa, the elapsing time from final descaling to coiling 15 to 40 seconds, the distance D from the nozzle to the steel sheet 150 to 250 mm, and the angle ⁇ formed between the nozzle and the sheet thickness direction of the steel sheet 5 to 10°.
  • the steel sheet After one second or more passes after the finish rolling process ends, the steel sheet is cooled to a coiling temperature of 400 to 700° C. at an average cooling temperature of 5° C./s or more. If the time from the end of finish rolling to the start of cooling is less than one second, recrystallization of austenite will be insufficient and anisotropy of the steel sheet will be actualized, and therefore it is not preferable for the aforementioned time to be less than one second.
  • the average cooling temperature from the end of finish rolling to the coiling temperature is less than 5° C./s, ferrite transformation will be promoted in a high temperature region and the micro-structure of the hot-rolled sheet will coarsen, and therefore it is not preferable for the aforementioned average cooling temperature to be less than 5° C./s.
  • the coiling temperature is more than 700° C., boride precipitation will be promoted and it will therefore be difficult to make the value of sol. Bq/B fall within the predetermined range in the final product sheet.
  • the coiling temperature is less than 400° C., because the strength of the hot-rolled sheet will increase excessively, there is a risk that the strength will impair the cold rolling properties in a subsequent cold rolling process.
  • Average thickness tsc of scale immediately after coiling of hot-rolled coil 3 ⁇ m or more
  • a method for measuring tsc is adopted in which a hot-rolled steel sheet is separately manufactured using the same chemical composition and the same hot rolling conditions prior to coiling, and coiling of the steel sheet is then performed at a temperature at which scale does not sufficiently grow after coiling, specifically, a temperature of 300° C. or less, and the thickness of the scale thereof is measured and adopted as the measurement value for tsc. 10 ⁇ 5 ⁇ Do ⁇ 10 ⁇ 3 (iii)
  • the hot-rolled coil after cooling may be subjected to pickling according to the normal method. Further, skin pass rolling may be performed to straighten the shape of the hot-rolled coil and improve the pickling property.
  • Cold rolling is performed on the steel sheet after the hot rolling described above.
  • the cold-rolling rate is made 20% or more.
  • the cold-rolling rate is made 80% or less.
  • the cold-rolling rate is preferably 30% or more, and is preferably 70% or less.
  • the steel sheet is subjected to annealing by means of a continuous annealing line.
  • the average heating rate in a temperature range from 650° C. to the Ac 3 point is made 10° C./s or less.
  • the aforementioned average heating rate is made 0.1° C./s or more.
  • the Ac 3 point (° C.) can be obtained by the following formula (v).
  • Ac 3 910 ⁇ 203C 0.5 ⁇ 15.2Ni+44.7Si+104V+31.5Mo ⁇ 30Mn ⁇ 11Cr ⁇ 20Cu+700P+400Al+400Ti (v)
  • each symbol of an element in the formula represents the content (mass %) of the relevant element contained in the steel, and in a case where the content is 0, 0 is substituted into the formula to perform the calculation.
  • the steel sheet After the temperature rises, the steel sheet is held for one second or more at a predetermined highest heating temperature.
  • a particular limitation is not set with respect to the upper limit of the holding time. However, the producibility of the steel sheet will be impaired if the holding time is too long, and hence it is preferable to make the 1000 seconds the upper limit value of the holding time. Further, upper and lower limits of the highest heating temperature may be appropriately selected in a range in which austenitization is caused to adequately progress.
  • the highest heating temperature is preferably made 720° C. or more, and more preferably is made 760° C. or more.
  • the highest heating temperature is preferably made the Ac 3 point+30° C. or less, and is more preferably made the Ac 3 point ⁇ 10° C. or less.
  • the highest heating temperature is preferably made the Ac 3 point ⁇ 30° C. or more, and is more preferably made the Ac 3 point or more.
  • the highest heating temperature is too high it will lead to damage of the heating furnace, and hence the Ac 3 point+100° C. is made the upper limit value thereof.
  • treatment is performed that includes a first cooling process from the highest heating temperature to a first cooling stopping temperature, a second cooling process from a second cooling starting temperature that is equal to the aforementioned first cooling stopping temperature to a second cooling stopping temperature, and a heat treatment process of holding the steel sheet in a predetermined temperature range.
  • the steel sheet is cooled from the highest heating temperature to a temperature (first cooling stopping temperature) of 750° C. or less at an average cooling temperature of 10° C./s or less.
  • the average cooling temperature is preferably 5° C./s or less.
  • the stopping temperature is preferably 700° C. or less, and more preferably is 650° C. or less.
  • the steel sheet is cooled from a second cooling starting temperature that is equal to the first cooling stopping temperature to a temperature (second cooling stopping temperature) that is not more than the Ms point. If the second cooling stopping temperature is more than the Ms point, it is difficult to make the proportion of tempered martensite to the martensite overall 50% or more, and there is a risk that the bendability will deteriorate.
  • the average cooling temperature is preferably made 10° C./s or more. If the average cooling temperature is less than 10° C./s, the area fraction of polygonal ferrite becomes excessive and there is a risk that the strength will decrease. Although it is not particularly necessary to define the upper limit of the average cooling temperature, it is preferable to make 300° C./s the upper limit because special facilities are required in order to realize a cooling rate that is more than 300° C./s.
  • Ms 550 ⁇ 361C ⁇ 39Mn ⁇ 35V ⁇ 20Cr ⁇ 17Ni ⁇ 10Cu ⁇ 5Mo+30A (vi)
  • each symbol of an element in the formula represents the content (mass %) of the relevant element contained in the steel, and in a case where the content is 0, 0 is substituted into the formula to perform the calculation.
  • a heat treatment is performed that holds the steel sheet in a temperature region of 200 to 400° C. for 10 seconds or more. If the aforementioned heat treatment temperature is less than 200° C. or the aforementioned holding time is less than 10 seconds, it will be difficult to make the proportion of tempered martensite to the martensite overall 50% or more, and there is a risk that the bendability will deteriorate. Further, if the aforementioned heat treatment temperature is more than 400° C., it will be difficult to secure strength because martensite will be excessively tempered. Although an upper limit of the holding time is not particularly defined, it is preferable from the viewpoint of productivity to make the upper limit of the holding time 1000 seconds or less.
  • electrogalvanization may be performed after the steel sheet has passed through the aforementioned continuous annealing line, or the steel sheet may be passed through a continuous hot-dip galvanization line.
  • the conditions for the common method may be followed with regard to the conditions for electrogalvanization.
  • the pre-plating temperature is less than 420° C., heat dissipation in the hot-dip galvanizing bath will increase and will hinder productivity. On the other hand, if the pre-plating temperature is more than 520° C., pearlite transformation will occur and it will therefore be difficult to obtain the desired steel micro-structure.
  • the time from cooling to the pre-plating temperature to immersion in the hot-dip galvanizing bath is not particularly defined, it is preferable from the viewpoint of productivity that the time is 100 seconds or less.
  • the alloying treatment temperature is made a temperature in the range of 460 to 580° C. If the alloying treatment temperature is less than 460° C., productivity will be hindered because a long time period will be required for the alloying reaction. On the other hand, if the alloying treatment temperature is more than 580° C., pearlite transformation will occur and it will therefore be difficult to obtain the desired steel micro-structure.
  • the steel sheet After immersion in the hot-dip galvanizing bath or after the alloying treatment, the steel sheet is cooled to a temperature (second cooling stopping temperature) that is not more than the Ms point. If the second cooling stopping temperature is more than the Ms point, it will be difficult to make the proportion of tempered martensite to the entire martensite 50% or more, and there is a risk that the bendability will deteriorate.
  • second cooling stopping temperature is more than the Ms point, it will be difficult to make the proportion of tempered martensite to the entire martensite 50% or more, and there is a risk that the bendability will deteriorate.
  • the average cooling temperature is preferably made 10° C./s or more. If the average cooling temperature is less than 10° C./s, the area fraction of bainite becomes excessive and there is a risk that the strength will decrease. Although it is not particularly necessary to define the upper limit of the average cooling temperature, it is preferable to make 300° C./s the upper limit because special facilities are required in order to realize a cooling rate that is more than 300° C./s.
  • a heat treatment is performed that holds the steel sheet in a temperature region of 200 to 400° C. for 10 seconds or more. If the aforementioned heat treatment temperature is less than 200° C. or the aforementioned holding time is less than 10 seconds, it will be difficult to make the proportion of tempered martensite to the entire martensite 50% or more, and there is a risk that the bendability will deteriorate. Further, if the aforementioned heat treatment temperature is more than 400° C., it will be difficult to secure strength because martensite will be excessively tempered. Although an upper limit of the holding time is not particularly defined, it is preferable from the viewpoint of productivity to make the upper limit of the holding time 1000 seconds or less.
  • the steel sheet After the heat treatment in the aforementioned (c-5) or (c-9), the steel sheet may be subjected to temper rolling for the purpose of flatness straightening and adjustment of the degree of surface roughness. In this case, to avoid a deterioration in ductility it is preferable to make the rate of elongation 2% or less.
  • R1 Total rolling reduction at 1050 to 1150° C.
  • R2 Total rolling reduction from 1050° C. or less to before final finishing pass
  • HR Average heating rate in temperature range from 650° C. to highest heating temperature
  • a JIS No. 5 tensile test specimen was taken from a direction (width direction) orthogonal to the thickness direction and the rolling direction of each of the obtained cold-rolled steel sheets, and a tensile test was performed in accordance with JIS Z 2241 and the tensile strength (TS), yield strength (YS) and total elongation (El) were measured.
  • TS tensile strength
  • YS yield strength
  • El total elongation
  • a test specimen with dimensions of 150 mm ⁇ 150 mm was cut out, and a “JFS T 1001 Hole Expansion Test Method” of the Japan Iron and Steel Federation Standards was performed to measure the hole expansion ratio ( ⁇ ).
  • a strip test specimen was cut out in an orthogonal direction (width direction) to the rolling direction and thickness direction, the V-bending test specified in JIS Z 2248 was performed while varying the bending radius, and a minimum bending radius R at which cracking did not occur was determined, and the bendability was evaluated based on a ratio (R/t) between the sheet thickness t of the cold-rolled steel sheet and the minimum bending radius R.
  • the area fraction of retained austenite was determined by calculating the area of a region having an FCC structure by the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method.
  • sol. Bs and sol. Bq were determined by calculating the B amount consumed as a precipitate by measuring the mass of borides in the steel by the electrolytic extraction residue method, and thereafter deducting the calculated B amount from the B content contained in the steel.
  • Non-Patent Document 1 was used as the method for determining the precipitated B amount by the extraction residue method.
  • V ⁇ Area fraction of polygonal ferrite
  • VTM Area fraction of tempered martensite
  • the results showed that the tensile strength was 980 MPa or more and the value of R/t was 2.5 or less, and the example embodiments thus had high strength and favorable bendability.
  • TS tensile strength
  • YiS yield strength
  • El total elongation
  • ⁇ hole expansion ratio
  • R/t ratio between the sheet thickness t and the minimum bending radius R, area fractions of the steel micro-structure, as well as the values of sol. Bs/B and sol. Bq/B for the obtained cold-rolled steel sheets were measured by the same methods as in Example 1.
  • the results showed that the tensile strength was 980 MPa or more and the value of R/t was 2.5 or less, and the example embodiments thus had high strength and favorable bendability.
  • TS tensile strength
  • YiS yield strength
  • El total elongation
  • ⁇ hole expansion ratio
  • R/t ratio between the sheet thickness t and the minimum bending radius R, area fractions of the steel micro-structure, as well as the values of sol. Bs/B and sol. Bq/B for the obtained cold-rolled steel sheets were measured by the same methods as in Example 1.
  • the results showed that the tensile strength was 1180 MPa or more and the value of R/t was 3.5 or less, and the example embodiments thus had high strength and favorable bendability.
  • steels A, B, C, D, F, I and J were melted in a laboratory and ingots were cast. Thereafter, hot rolling was performed under the conditions shown in Table 15, and hot-rolled steel sheets having a thickness of 2.0 to 3.0 mm were obtained. Note that, the various conditions in the descaling process were the same as in Example 1. Thereafter, pickling was performed, followed by cold rolling with the rolling reductions shown in Table 16 to obtain cold-rolled steel sheets having a thickness of 1.0 mm. The obtained cold-rolled steel sheets were subjected to a heat treatment that simulated a continuous hot dip galvanization line under the conditions shown in Table 16.
  • TS tensile strength
  • YiS yield strength
  • El total elongation
  • ⁇ hole expansion ratio
  • R/t ratio between the sheet thickness t and the minimum bending radius R, area fractions of the steel micro-structure, as well as the values of sol. Bs/B and sol. Bq/B for the obtained cold-rolled steel sheets were measured by the same methods as in Example 1.
  • the results showed that the tensile strength was 1180 MPa or more and the value of R/t was 3.5 or less, and the example embodiments thus had high strength and favorable bendability.
  • a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet that are excellent in bendability can be obtained.

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